Tom Carroll pulls up the birdsong application on his iPhone, summons the call of a song sparrow and points the device to the meadow that starts about 20 feet from his home.

Sure enough, after a minute or two, an actual song sparrow starts calling back to the digital one. This might be little more than an endearing party trick if Carroll and his wife, Dora Marcus, lived on the edge of a field in, say, upper Montgomery County. But they live a little farther downstream on the Potomac River, in the shadow of Key Bridge. The meadow, perhaps Washington's greatest secret garden, is perched atop a Pepco electrical substation on the Georgetown waterfront.

The straw-colored flower heads of a grass called calamagrostis are just visible for the most observant of motorists on the Whitehurst Freeway, and pedestrians on Key Bridge might glance back to see the fuzzy-topped brick substation, but there is no real public vantage point to view the full sweep of this prairie in the sky. This paradox is heightened during rush hour, when the freeway, Key Bridge and M Street NW are filled with weary drivers unaware of the meadow floating above them.

The grasses give movement in the slightest breeze: The calamagrostis sways like pennants on wires on the meadow's southern edge, and the molinia stems shimmer veil-like on its western side. From high summer into fall, the full meadow goes through subtle color gyrations.

For Carroll and Marcus and their neighbors, the garden presents layers of color and texture, and a kinetic quality augmented by the birds. Mallard ducks, doves, sparrows, goldfinches and other species find both refuge and food here. In this 10,000 square feet of meadow 42 feet in the air, they thrive unmolested by cats, raccoons, people, snakes or floods. Only landscapers have access to the plants, and they must use long ladders. "Nobody picks the flowers," said Marcus, chuckling. And yet from their fifth-floor, two-bedroom condo, it's almost as if you can reach out and stroke the grasses.

Look to the right, and the meadow forms the ground plane for a view of Key Bridge with the Rosslyn skyline behind. Look downriver, and the garden frames a vista of the Potomac curving before the Watergate and the Kennedy Center. With the balcony door open, the apartment is filled with the whooshing of cars on the Whitehurst Freeway and the growl of jets twisting their way to Reagan National Airport. Close the door, with its soundproofed glass, and the scene shifts to one of surreal silence. The calamagrostis continues to dance.

Marcus and Carroll live in the plush condo building known only by its address, 3303 Water Street. The nine-story building is sandwiched between the freeway and the C&O Canal, and contains 72 apartments. It looks like a converted factory or warehouse but is only five years old and stands on a site that used to be occupied by electrical transformers and a canalside hill.

On a once archly utilitarian waterfront that had been slowly redeveloped over the past 25 years, this key parcel "was the missing tooth," said developer Anthony Lanier. Urban infill projects bring transformational architecture, but they also have their obstacles. Here, the hulking brick substation stood between the condo and waterfront, and the well-heeled owners of south-facing condos on and above the fifth floor would have peered down on the substation's expansive pitch and gravel roof.

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The idea of green roofs is still a relatively new concept in the United States but is well known to Lanier, who grew up in Austria. His company, Eastbanc, purchased the rights to install a green roof on the substation, after considering but rejecting the idea of building the condo so that it would overhang the substation. The roof garden, said Lanier, was the device "that would get the eye to the river in the most pleasant way."

Engineers for Pepco were cautious about the project and worried that a rooftop garden, with its irrigation system and capacity to capture large amounts of rainwater, might leak. Leaks into the substation, with its huge transformers that reduce high-voltage electricity for distribution to the neighborhood, would present potential peril.

"We took all the gravel off the roof," said Lisa Delplace, one of the designers at the Washington-based firm of Oehme, van Sweden & Associates tackling this and the other landscape architecture requirements for the project, which was begun in 2001. A thick and seamless rubbery membrane was then laid on the bare concrete roof, and the meadow was built on top of that.